Do you have it set to automatically back-up your system? If you do, then it's most likely creating an image of your OS and ALL of your data and system files that's on your computer. That way if your entire system crashes, you have a back-up of your system the way it was before it went corrupt. Now that I think about it, if you got the HDD and you installed the software that came with it, then that's probably what it is. A lot of times, the software will tell Windows to automatically back-up your system, by default. Honestly, it could be any number of things, but creating an image is what it sounds like to me. Without doing any kind of troubleshooting and going at this blindly. Oh and if you access the drive, and you don't immediately see anything out of the norm, then it's most likely that the files are hidden, which would lead me to believe that they are system folders/files. Since I don't know which OS you are running on, I would do a Google search for "unhide files and folders" and then whatever OS you are running.
